I use an two different LG boxes to QAM and ATSC stuff. Both I believe are discontinued now, but on ebay you can probably find them
LG
LST-4200a (does NTSC, ATSC, QAM)
LST-3510a (ATSC, QAM, Upconverting DVD player)
Both get my charter cable QAM signals that are in the clear. Which are locals in HD among other things. Each system is different, so who knows what you'll pick up.
Also note that the QAM channels will not normally match what the provider lists the channel number as. Most of the QAM capably boxes scan up to channel 135, but your cable stb remaps the channels to the ones you see on the channel list.
I tried using a Samsung T-451 originally. It sucked, would not do QAM256, only QAM64. Of course this kind of spec is not on the box. Just my experience, maybe the new samsung stuff does more.